Age-related fine structural changes in axons and synapses during deafferentation of the rat pyriform cortex: a possible basis for plasticity.

The purpose of this study was to compare the sequence of axonal and synaptic alterations following deafferentating lesions at selected postnatal ages and relate these changes to synaptic organization in the olfactory cortex. Rats received unilateral olfactory bulb ablation at 2 1/2, 6, 9 and 13 days of age and were studied at survivals of 12 h to 30 days. At least three clearly different forms of acute degeneration were seen; flocculent, granular and dense with the granular form an intermediate form. The proportion of granular and especially dense degeneration increases after six days of age as does the presence of glia. The denser the type of degeneration, the greater the retention of remnants of this form of synaptic degeneration at deafferented postsynaptic sites. This as well as the increased presence of glia after six days may be important factors in the limitation of plastic reorganization or reinnervation in more mature individuals. The youngest operated animals show rapid vacating of the receptor site, relative absence of glia and striking evidence of competitive reoccupation of deafferented sites.